Bacterial associations

Question 1

Define carriage?

Answer 1

Establishment of a long term harmless relationship

Question 2

Define pathogenicity?

Answer 2

Ability to cause disease

Question 3

Define virulence?

Answer 3

Severity of the disease caused

Question 4

Define commensal?

Answer 4

An animal or plant which lives attached to or as a tenant of the other, and both share their food.

Question 5

Define symbiosis?

Answer 5

Association of two different organisms which live attached to or as a tenant of the other and contribute to each other’s support

Question 6

What is syntrophy/origin of eukaryotes?

Answer 6

Highly nutritionally interdependent (syntrophic) communities of eubacteria and archaea exist in anoxic environments

Patterns of gene exchange among members of these communities suggest that they evolved as communities rather than independent organisms

May be that the eukaryotic cell originated from such interactions

Question 7

Stages of host adaptation? (Genome getting smaller)

Answer 7

1. Free living and extracellular: everything happens outside of the genome, gene loss
2. Facultative intracellular: gene loss occurs
3. Obligate intracellular: gene loss, transfer to host genome
4. Obligate intracellular mutualist: no longer recombination, or genomic island, or phage, or plasmid
5. Organelle: gene loss, transfer to host genome occurs in the genome

Question 8

What are diatoms?

Answer 8

Abundant microscopic algae and contribute about 20% global photosynthesis

Associated with proteobacteria, and bacteriodetes. Bacteria contribute to diatom genomes via HGT – recruitment of metabolic capacity

Question 9

Describe the lichen mutualism?

Answer 9

The mycobiont (fungal constituent of a lichen) protects the photobiont and absorbs mineral nutrients

The prokaryotic photobiont (photosynthetic part of a lichen) synthesises organic nutrients and fixes nitrogen

Multiple combinations of mycobiont and photobionts are possible – not a tight association

Question 10

Describe the Rhizobium interaction?

Answer 10

Widespread & important biological symbiosis
Species specific with the bacteria and the plants

Plant: provides nutrients
Bacterium: fixes nitrogen

• > formation of Rhizobium root nodules:
  1. Recognition and attachment
  2. Signalling
  3. Invasion
  4. Travel through infection thread
  5. Bacteriod formation
  6. Bacterial and plant growth to form the nodule

Question 11

Insect endosymbionts?

Answer 11

Buchnera:

Obligate intracellular endosymbiosis of aphids, without the bacteria, which are maternally transmitted, the aphids die.

The bacteriocytes are intracellularly located

Host role: supplies energy, carbon, and nitrogen, in the form of glutamine from phloem

Symbiont: production of amino acids, especially tryptophan

Coevolution of the bacteria and host, apparent in the similar aphid & symbiont phylogenetic trees

Question 12

Human microbiota?

Answer 12

More microbial cells than human cells in the human body. There are a variety of environments in terms of temperature, oxygen acidity, and host defence.

Highly diverse with variation among, but stability within, individuals

Lifestyle is probably important

Presence of professional pathogens is rare in the normal microbiota

Question 13

When does disease in animals occur?

Answer 13

1. Microbes have an effect on host biology, e.g. cancer or immunological diseases
2. Normal microbiota invade, e.g. accidental pathogens which don’t gain from destroying host, bacteria gain virulence factors
3. Other microorganisms invade, e.g. obligate professional pathogens, and opportunistic pathogens (can cause disease when the host’s resistance is low – normally a commensal)

Question 14

Microbiome and cancer links?

Answer 14

Can promote cancers by a number of specific mechanisms, e.g. promotion of inflammation by Helicobacter pylori

Human/Microbe “super organism” can become disrupted, with cancer caused by: immune response; dysbiosis; genotoxicity; and metabolism

Question 15

What is septic shock?

Answer 15

Sepsis develops when the chemicals the immune system releases into the bloodstream to fight an infection cause inflammation throughout the entire body instead
Toxic shock

Question 16

Outline main features of diptheria?

Answer 16

Role of toxins (poisonous products of organisms, not capable of reproducing themselves)

Was a major childhood infection worldwide.

Toxin is essential for disease and is encoded on a phage, a classic virulence factor

Neither the phage or bacterium require the toxin for host colonisation

High levels of immunisation can control the disease, however, unimmunised populations represent reservoirs of infection requiring continued high immunisation colonisation

Question 17

What are toxins?

Answer 17

Bacteria have evolved a wide variety of secreted products, especially protein toxins which attack and damage the host or act to modulate/control signalling systems

e.g. to regulate the host immune response.

Question 18

An example of a commensal lifestyle bacteria?

Answer 18

Neisseria meningitides:

Common inhabitant of the mucosa of young adults.

Aerosol transmission is followed by transitory colonisation (months-years).

Occasionally causes disease, but doesn’t contribute to transmission

Pilus contracts to pull close to cell. Growth and immune evasion. Invasion of the host cells. Release into bloodstream (blood is a good growth medium), then more growth.

Release of endotoxin leads to septicaemia (blood poisoning especially that caused by bacteria/their toxins); or invasion of meninges leads to meningitis.

Question 19

Example of an opportunistic pathogen?

Answer 19

Vibrio cholera

1. Ingestion via food/drink
2. Infection leads to 3, 4, or 5
3. Excretion leads to 6
4. Death
5. Recovery
6. Environmental growth leads to 1

A gram-negative bacterium, common in water

Only two of the 200 serotypes are commonly associated with epidemic disease

All pathogenic cholera express cholera toxin and toxin co-regulated pilus

Question 20

Define obligate?

Answer 20

Restricted to a particular function or mode of life.

Question 21

Example of an obligate pathogen?

Answer 21

Mycobacterium tuberculosis:

33% of the world’s population are carriers, without disease activation

Transmission via milk or aerosol transmission

Pulmonary tuberculosis:

1. Breakdown of the latent stage of infection may be caused by stress/age, coinfection with HIV, etc.
2. Release of mycobacteria throughout the body – immune system gets overwhelmed
3. Proliferation in the lungs, severe lung damage (cavity formation, caused by host immune responses)
4. Highly infectious to others